The field of the invention is the recovery of catalysts used in the synthetic production of higher fatty esters.
The state of the art of the recovery of cobalt catalysts used in hydrocarboxylation may be ascertained by reference to U.S. Pat. Nos. 3,310,600; 3,507,891; 3,856,832; 3,906,016; and 4,041,057, the disclosures of which are incorporated herein.
It is known in the prior art that fatty acids can be prepared by reacting olefins with carbon monoxide and a compound containing a replaceable hydrogen atom, such as for instance, an alkanol, in the presence of a catalyst containing a metal of Group VIII of the Periodic Table of the elements and possibly containing also a promoter as disclosed by J. Falbe in "Synthesen mit Kohlenmonoxide" (Synthesizing with Carbon Monoxide), Springer publishers, Berlin, Heidelberg, New York, 1967.
A preferred embodiment of this reaction, which is termed hydrocarboxylation, is the reaction in the presence of cobalt catalysts. An especially preferred embodiment comprises the additional use of pyridine or of a non-ortho-substituted alkylpyridine as the promoter.
A substantial limitation of this homogeneously catalyzed reaction is the recovery of the relatively costly cobalt from the reaction mixture in a form permitting its renewed use as a catalyst.
A method described in West German Published Application No. 19 63 804 solves this problem by recovering the cobalt catalyst as the distillation sump substance and feeding it back as such into the reaction. Such a method, however, leads to losses of cobalt due to the separation of metallic cobalt in the reprocessing by distillation.
Such losses can be avoided in the method disclosed by U.S. Pat. No. 3,507,891 provided that the cobalt-catalyzed hydrocarboxylation is carried out in the presence of an alkylpyridine acting in a stabilizing manner in lieu of pyridine.
On the other hand, U.S. Pat. No. 3,906,016 discloses that such a method results neither in complete cobalt recovery nor does it make it possible to recycle the catalyst that would be free of degraded activity loss. The method of U.S. Pat. No. 3,906,016 avoids a separation of the metallic cobalt in the distillation reprocessing by carrying out the cobalt/pyridine or cobalt/alkylpyridine catalyzed reaction of olefin with alkanol and carbon monoxide in the presence of small amounts of water (0.1 to 2 moles/mole of olefin). In addition to esters, the corresponding fatty acids are thus formed in slight amounts. The conditions of reprocessing the reaction mixture by distillation leads to the formation of fatty acid cobalt compounds. Such thermally stable cobalt compounds are also formed when the reaction mixture is subjected to oxidizing treatment with air or oxygen before reprocessing.
Such a method, however, also has its limitations. Thus, the formation of fatty acids, which is constrained by the presence of water, leads to losses in the yield of the desired esters (the ester yields in part may drop down to 67%). The presence of water and fatty acids under the drastic conditions of hydrocarboxylation moreover leads to corrosion problems. Again, when paraffin diluted olefins are used, for instance, those prepared industrially by chlorinating/dechlorinating paraffins, the presence of water during the reaction results in demixing and hence to slowing the reaction. Lastly, this method also fails to make possible a complete conversion of the cobalt used into the desired fatty acid salt.
U.S. Pat. No. 3,856,832 discloses a further method for catalyst recovery. The process of this patent takes place in the presence of an excess of methanol as the esterification component, and a hydrocarbon is added to the reaction mixture at the end of reaction. In this manner, two phases are formed, of which the lowermost contains at least 95% of the cobalt used as catalyst. The lowermost phase containing the cobalt in active form then can be fed directly back into the hydrocarboxylation reaction.
The limitations of this method of U.S. Pat. No. 3,856,832 on the one hand consist in the restriction of the process to the production of methylesters, and on the other hand in the loss of the cobalt contained in the uppermost phase (up to 5% by weight).
In the method of U.S. Pat. No. 4,041,057, the cobalt remaining in the uppermost phase can be recovered by the process of U.S. Pat. No. 3,856,832 by burning the tarry residue obtained after its distillation reprocessing at temperatures from 1,000.degree. to 4,000.degree. F., by filtering the cobalt oxide so formed out of the combustion gases and by converting the cobalt oxide by means of carbon monoxide and hydrogen into cobalt carbonyl. This cobalt carbonyl can be fed back into the hydrocarboxylation.
While a complete recovery of the cobalt is possible with the combination of methods of U.S. Pat. Nos. 3,856,832 and 4,041,057, the profitability of such a procedure, however, is substantially reduced by the use of large amounts of hydrocarbons that appreciably affect the distillation reprocessing and by a costly, multistage recovery of the residual cobalt in the upper phase.